The present invention relates to electric motors and in particular to an induction motor having an independently rotating permanent magnet rotor variably coupled to an inductive rotor to reconfigure the motor from asynchronous induction operation at startup to synchronous operation after startup for efficient operation.
A preferred form of electric motors are brushless AC induction motors. The rotors of induction motors include a cage (or squirrel cage resembling a “hamster wheel”) rotating inside a stator. The cage comprises axially running bars angularly spaced apart on the outer perimeter of the rotor. An AC current provided to the stator introduces a rotating stator magnetic field inside the rotor, and the rotating field inductively induces current in the bars. The current induced in the bars creates an induced magnetic field which cooperates with the stator magnetic field to produce torque and thus rotation of the rotor.
The introduction of current into the bars requires that the bars are not moving (or rotating) synchronously with the rotating stator magnetic field because electromagnetic induction requires relative motion (called slipping) between a magnetic field and a conductor in the field. As a result, the rotor must slip with respect to the rotating stator magnetic field to induce current in the bars to produce torque, and the induction motors are therefore called asynchronous motors.
Unfortunately, low power induction motors are not highly efficient at designed operating speed, and are even less efficient under reduced loads because the amount of power consumed by the stator remains constant at such reduced loads.
One approach to improving induction motor efficiency has been to add permanent magnets to the rotor. The motor initially starts in the same manner as a typical induction motor, but as the motor reached its operating speed, the stator magnetic field cooperates with the permanent magnets to enter synchronous operation. Unfortunately, the permanent magnets are limited in size because if the permanent magnets are too large, they prevent the motor from starting. Such size limitation limits the benefit obtained from the addition of the permanent magnets.
U.S. patent application Ser. No. 14/151,333 filed Jan. 9, 2014 filed by the present Applicant discloses an electric motor having an outer stator, an inner rotor including bars, fixed to a motor shaft, and a free spinning outer rotor including permanent magnets and bars, residing between the inner rotor and the stator. At startup, a rotating stator field accelerates the free spinning outer rotor, and after accelerating, the permanent magnets of the free spinning outer rotor accelerate and then lock with the inner rotor to achieve efficient permanent magnet operation.
The design of the '333 application is suitable for some motor designs, but in other designs, surface effects on the surface of the inner rotor reduce or prevent coupling of the inner rotor with the rotating magnetic fields.